There are many high voltage electrical systems that require switching. These systems may switch voltages that are much higher than typical logic and control circuitry voltages and may include 20V, 50V, 100V, 200V, or even higher voltages. Semiconductor switches are often used to provide power switching to such electrical systems. These semiconductor switches need to be designed to accommodate the voltages needed in the electrical system. These semiconductor switches may have an on resistance Ron and a breakdown voltage BV.
High-voltage MOS transistors that may be made in a certain semiconductor processes obey the relationship:RonA=BV  (1)Ron is the transistor's resistance in the on-state, which should be as small as possible. A is the transistor's area, is a constant that depends on the process and geometry details of the transistor, and BV is the breakdown voltage of the transistor, which should be as large as possible. For example, for a vertical transistor (VDMOS), =2.5, and for optimal lateral transistors (LDMOS), =2.33. (See Zing, ON THE SPECIFIC ON-RESISTANCE OF HIGH-VOLTAGE AND POWER DEVICES, IEEE Trans. El. Dev. p. 492, 2004). Accordingly a lower value of is desirable if a low RonA product is desired at a given breakdown voltage.
This relationship demonstrates two issues with semiconductor switches. First, transistors with a large breakdown voltage BV require a large area. Doubling the breakdown voltage while keeping Ron the same requires an area that is 5-5.7 times larger for the transistor. This larger area increases the size and the cost of the semiconductor switch.
Second, each desired transistor breakdown voltage requires a specific transistor optimization in terms of length of the drift region, doping profile, and gate location. Accordingly, this requires that a range of different transistors sizes need to be developed, e.g., 20V, 60V and 100V transistors. Each of these transistors needs to be designed, qualified and modeled. Because the spacing between available voltages is large, suboptimal may designs result. For example if only 20V, 60V, and 100V transistors are available for a 30V application, the 60V transistor will be used by the designers, thus requiring an area 5 times larger than a dedicated 30V transistor would yield.
Third, breakdown voltages beyond what is offered by the technology are not possible. For example, an application requiring 150V or 200V breakdown voltage may not be possible or the transistor area and cost required are prohibitive.